1. Field of the Invention
This invention relates to a color cathode-ray tube mainly for multicolor display of characters and graphics.
2. Description of the Prior Art
Generally, glasses that are used as face plates for cathode-ray tubes are classified into a clear face whose light transmittance in the visible region is 75% or above, a gray face with a light transmittance of 60-75%, and a tint face with a light transmittance of 60% or below. That is, these types of glass are classified by light transmittance.
Heretofore, in the case of color cathode-ray tubes, there has been a tendency to attach more importance to brightness than to contrast. For this reason, the gray face or clear face, which is superior in light transmittance, has been frequently used. On the other hand, in order to absorb ambient light and increase contrast, it is advantageous to use the tint glass, which is low in light transmittance. However, the optical output of color cathode-ray tubes is not generally so strong as to provide sufficient light even if the light transmittance of the glass is lowered. Therefore, it is usual practice to use the clear face or gray face, which is high in light transmittance. In this case, a black matrix type construction is employed.
The curve F1 shown in FIG. 1 indicates the light transmittance-wavelength characteristic of the usual clear face, and F2 indicates the light transmittance-wavelength characteristic of the gray face. It is understood from FIG. 1 that the face plates made of these glasses have substantially flat transmittance characteristics in the visible region.
On the other hand, cathode-ray tubes for color display used to display characters and graphics are very small in light emitting surface area as compared with cathode-ray tubes which are generally used in color television. Therefore, contrast becomes a very important factor, and it is preferable to make the color of the face surface blacker. Thus, as disclosed in U.S. Pat. No. 3,143,683, it is proposed to provide light wavelength selectivity in the light transmittance characteristic of the face plate by adding 0.3-1.5% by weight of a rare earth metal such as neodymium when glass is melted for production of the face plate.
FIG. 2 shows the light transmittance-wavelength characteristic of a 10 mm thick glass sheet having about 1% by weight of neodymium in the form of Nd.sub.2 O.sub.3 incorporated therein. As is clear from FIG. 2, this glass has large light absorption bands in wavelength regions of about 570-590 nm and 510-530 nm. The light absorption bands in these regions are in the wavelength regions corresponding to the valleys of the emission spectrums of usual red, green and blue phosphors. Therefore, the face plate made of this glass well transmits light in the light emitting wavelength region of each of the red, green and blue fluorescent bodies and well absorbs light in the other wavelength regions. As a result, the contrast of the picture can be improved without decreasing brightness so much, and it is considered possible to greatly increase the chromaticity of each of the primary colors red, green and blue for the purpose of the filter effect of these light absorbing bodies.
Thus, on the basis of this technique, improvements in face glass have been made to provide a cathode-ray tube capable of producing a more easily visible picture. That is, it has been tried to increase contrast while suppressing the light transmittance in the visible region by adding slight amounts of such colorants as chromium, nickel and cobalt in the form of Cr.sub.2 O.sub.3 amounting to 100 ppm, NiO amounting to 100 ppm, and Co.sub.3 O.sub.4 amounting to 8-9 ppm. The glass containing such colorants has a characteristic as indicated by a curve A in FIG. 3. Thus, it has become possible to provide a color cathode-ray tube which is excellent in contrast and which is easy to watch.
However, this color cathode-ray tube is excellent in contrast, when combined with phosphors of relatively short persistence called P 22 in the EIA Standard and used for ordinary color television (for example, red=Y.sub.2 O.sub.2 S:Eu, green=ZnS:Au, Cu, Al, blue=ZnS:Ag), has performance excellent in both brightness and contrast. However, in the case of color display, since the picture is almost a stationary one, the flicker becomes a serious problem depending upon the recurrency frequency and quality of the displayed picture. For this reason, for color display use, the often used phosphors which emit green and red light, excluding blue, causing a relatively unobtrusive flicker, are of the long persistence nature, including Zn.sub.2 SiO.sub.4 :MnAs and (ZnMg).sub.3 (Po.sub.4).sub.2 :Mn. Of these, green Zn.sub.2 SiO.sub.4 and MnAs, which are called P 39 in the EIA Standard, have been improved in accordance with recent increasing demands, achieving a degree of brightness which, though not satisfactory, is almost practical. However, concerning red, the light emitting efficiency is low and there is no phosphor which provides sufficient brightness, so that it has been usual practice to use a method of increasing irradiation electron beams to bring them closer to practical brightness, if only to some extent: nevertheless, problems remain as to such points as degradation of the focus characteristics and brightness life of phosphors due to their use under large currents.